Telecommunications is one of the most important forces that drive the economic development of this world. Various telecommunication services (e.g., voice, electronic mail, facsimile, etc.) help people in all parts of the world to communicate almost instantaneously. When information can be exchange reliably, quickly and inexpensively, the efficiency of many organizations and business transactions improve drastically.
As a result of the needs for global communication, telecommunication services have evolved rapidly from a collection of separate, and largely incompatible, systems towards a universal network, in which a wide variety of services are integrated using a common digital form of transmission. The way to build this universal network is for all systems to follow a set of standards. One of the most important standard-setting bodies is the International Telecommunications Union ("ITU"). The part of the ITU responsible for setting standards is the ITU Telecommunications Standardization Sector ("ITU-T"), which used to be known as the International Telegraph and Telephone Consultative Committee ("CCITT"). Many of the standards referred to in this specification were set by CCITT.
It is known that over any given communication route, the most cost-effective transmission of telecommunication signal will, in general, be achieved by multiplexing as many communication channels as possible using a device called multiplexer. The multiplexed signal can be transmitted over a single communication link using a high signalling rate. As a result, a single link (which could be a electric wire, optical fiber, microwave transceiver, etc.) can carry communication signals of hundreds of users.
Transmission networks in the public switched telephone network, therefore, are designed around hierarchies of transmission rates, corresponding to increasing numbers of channels conveyed on a single multiplexed link. These hierarchies are defined in national and international standards (such as the CCITT). In Europe the hierarchy is based on a 30-channel 2048 kilo-bits per second ("kbps") primary rate (called a "E1" rate), while in the United States and Japan it is based upon a 24-channel 1544 kbps rate (called a "T1" rate). Examples of other rates are the 8448 kbps E2 rate (corresponding to four E1 channels), 34368 kbps E3 rate (corresponding to four E2 channels), and 139264 kbps E4 rate (corresponding to four E3 channels). The interconnection between successive multiplexers or to and from transmission systems are made using standardized interfaces, defined by CCITT in recommendation G.703. The signal structure at these interfaces is also standardized. As a result, telecommunications companies can build and reconfigure the transmission network on a modular basis, with different modules purchased from different vendors.
The CCITT recommendation allows for bit-rate tolerances at each level of the hierarchy. To maintain signals within these tolerances, each multiplexer contains a clock source. At higher order multiplexers, the incoming ("tributary") signals are originated at lower order multiplexers, each with its own clock source. As a result, the tributaries each has a different bit rate, although all should be within the specified tolerance. Such signals, having nominally the same frequency but differing within a defined tolerance, are called "plesiochronous" signals. The digital hierarchy involving these signals is called the "plesiochronous digital hierarchy" ("PDH").
One of the most important communication media is fiber optic because it can transmit signals having an extremely high signalling rate and is immune to many sources of noise. Optical Link Integrated Multiplexing Terminals (or "OPTIMUX") are major building blocks for building fiber optical based telecommunication infrastructure. Because most digital interfaces used internationally are based on the G.703 E1 standard, one of the most popular product categories in this area is the PDH multiplexing and transmission devices. These OPTIMUX products multiplex E1 signals into higher bit rates (e.g., E2 and E3 rates) according to PDH multiplexing hierarchy governed by G.742 and G.751 and directly convert the resultant higher tributary data streams into optical signals for transporting over optical fiber links. They are currently the most cost effective and easy to maintain transmission products on the market today.
As a result of the plesiochronous nature of the PDH, there is a need to synchronize the tributary signals. Typical methods involve adding justification or stuffing bits to the data streams. These additional bits occupy valuable bandwidth of a network. Further, complicated circuits are needed to process these bits and perform synchronization. In addition, the hierarchy must be followed rigidly (e.g., a multiplexed signal is generated from four tributary signals having the same rate which is one level lower in the hierarchy). Consequently, there is a need to design a system which can overcome these problems.